Sb Sites Research Articles

Enhancement of the thermoelectric properties in doped FeSb2 bulk crystals

Kondo insulator FeSb2 with large Seebeck coefficient would have potential in thermoelectric applications in cryogenic temperature range if it had not been for large thermal conductivity κ. Here we studied the influence of different chemical substitutions at Fe and Sb site on thermal conductivity and thermoelectric effect in high quality single crystals. At 5% of Te doping at Sb site, thermal conductivity is suppressed from ∼250 W/Km in undoped sample to about 8 W/Km. However, Cr and Co doping at Fe site suppresses thermal conductivity more slowly than Te doping, and even at 20% Cr/Co doping the thermal conductivity remains ∼30 W/Km. The analysis of different contributions to phonon scattering indicates that the giant suppression of κ with Te is due to the enhanced point defect scattering originating from the strain field fluctuations. In contrast, Te-doping has small influence on the correlation effects and then for small Te substitution the large magnitude of the Seebeck coefficient is still preserved, leading to the enhanced thermoelectric figure of merit (ZT ∼0.05 at ∼100 K) in Fe(Sb0.9Te0.1)2.

Electronic, vibrational, and transport properties of pnictogen-substituted ternary skutterudites

First principles calculations are used to investigate electronic band structure and vibrational spectra of pnictogen-substituted ternary skutterudites. We compare the results with the prototypical binary composition CoSb${}_{3}$ to identify the effects of substitutions on the Sb site, and evaluate the potential of ternary skutterudites for thermoelectric applications. Electronic transport coefficients are computed within the Boltzmann transport formalism assuming a constant relaxation time, using a methodology based on maximally localized Wannier function interpolation. Our results point to a large sensitivity of the electronic transport coefficients to carrier concentration and to scattering mechanisms associated with the enhanced polarity. The ionic character of the bonds is used to explain the detrimental effect on the thermoelectric properties.

Exit wave reconstruction from focal series of HRTEM images, single crystal XRD and total energy studies on SbxWO3+y(x∼ 0.11)

A new tungsten bronze in the Sb—W—O system has been prepared in a solid state reaction from Sb 2 O 3 , WO 3 and W metal powder. The average structure was determined by single crystal X-ray diffraction. Sb x WO 3+ y ( x ∼ 0.11) crystallizes in the orthorhombic space group Pm 2 1 n (no. 31), a = 27.8135(9) Å, b = 7.3659(2) Å and c = 3.8672(1) Å. The structure belongs to the ( n )-ITB class of intergrowth tungsten bronzes. It contains slabs of hexagonal channels formed by six WO 6 octahedra. These slabs are separated by three layers of WO 6 octahedra that are arranged in a WO 3 -type fashion. The WO 6 octahedra share all vertices to build up a three-dimensional framework. The hexagonal channels are filled with Sb atoms to ∼80% and additional O atoms. The atoms are shifted out of the center of the channels. Exit-wave reconstruction of focal series of high resolution-transmission-electron-microscope (HRTEM) images combined with statistical paramäeter estimation techniques allowed to study local ordering in the channels. Sb atoms in neighbouring channels tend to be displaced in the same direction, which is in agreement with total energy calculations on ordered structure models, but the ratio of the occupation of the two possible Sb sites varies from channel to channel. The structure of Sb x WO 3+ y exhibits pronounced local modulations.

Crystal structure and bonding characteristics of In-doped β-Zn4Sb3

The effects of indium impurity on the crystal structure and bonding characteristics of In-doped β-Zn4Sb3 were investigated by powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The XRD Rietveld refinement indicates that the indium impurity preferentially substitutes one of Sb atoms in Sb–Sb dimer at the 12c Sb(2) site and simultaneously leads to the increase of Zn occupancy. The observations of binding energy shift and a new valence state in Sb 3d core-level XPS spectra can be attributed to the charge transfer from In and Zn to Sb. As a result, more electropositive Zn atoms are needed to maintain the charge balance. The reduction of the lattice thermal conductivity is ascribed to the formation of the asymmetric Sb–In bond, resulting in much low lattice thermal conductivity of 0.49W−1K−1 of Zn4Sb2.96In0.04.

THE CRYSTAL STRUCTURE OF GUETTARDITE, PbAsSbS4, AND THE TWINNITE-GUETTARDITE PROBLEM

Guettardite from the Barika Au–Ag deposit in Azarbaijan Province, western Iran, formed in fractures developed in silica bands situated in massive banded pyrite and barite ores. Fractures host veinlets that contain a number of Ag–As–Sb–Pb-rich sulfosalts, tetrahedrite–tennantite, realgar, pyrite and Au–Ag alloy. The variation in the chemical composition of guettardite is minor: Pb 0.95–0.96 Sb 0.96–1.02 As 1.03–1.06 S 3.99–4.02 . The lattice parameters were determined from a single crystal as a 8.527(4), b 7.971(4), c 20.102(10) A, β 101.814(7)°, space group P 2 1 / c. The structure of guettardite contains six distinct coordination polyhedra of cations. Atoms Pb1 and Pb2 form slightly skewed tricapped trigonal coordination prisms arranged in a zig-zag layer. Two distinct As and two Sb sites in a chess-board arrangement form MeS 5 pyramids with trapezoidal bases. Guettardite is a homologue of sartorite, ideally PbAs 2 S 4 , and is the N = 3 member of the sartorite homologous series of sulfosalts. We contend that guettardite and twinnite are configurational polytypes composed of two alternating types of OD layers, formed by different orientations of tightly bonded crankshaft chains in adjacent As–Sb-based OD layers (layer symmetry P 1) separated by a Pb-based OD layers [layer symmetry Pm 2 1 ( n )]. Whether the observed differences in the Sb:As ratio determine the polytype is still an open question. A complete structural analogy has been found in the pair BaSb 2 S 4 – BaSb 2 Se 4 .

First-principles description of anomalously low lattice thermal conductivity in thermoelectric Cu-Sb-Se ternary semiconductors

Experimental measurements have recently shown that Cu${}_{3}$SbSe${}_{3}$ exhibits anomalously low and nearly temperature-independent lattice thermal conductivity, whereas Cu${}_{3}$SbSe${}_{4}$ does not exhibit this anomalous behavior. To understand this strong distinction between these two seemingly similar compounds, we perform density functional theory calculations of the vibrational properties of these two semiconductors within the quasiharmonic approximation. We observe strikingly different behavior in the two compounds: almost all the acoustic-mode Gr\"uneisen parameters are negative in Cu${}_{3}$SbSe${}_{4}$, whereas almost all are positive in Cu${}_{3}$SbSe${}_{3}$ throughout their respective Brillouin zones. The average of the square of the Gr\"uneisen parameter for the acoustic mode in Cu${}_{3}$SbSe${}_{3}$ is larger than that of Cu${}_{3}$SbSe${}_{4}$, which theoretically confirms that Cu${}_{3}$SbSe${}_{3}$ has a stronger lattice anharmonicity than Cu${}_{3}$SbSe${}_{4}$. The soft frequency and high Gr\"uneisen parameters in Cu${}_{3}$SbSe${}_{3}$ arise from the electrostatic repulsion between the lone ${s}^{2}$ pair at Sb sites and the bonding charge in Sb-Se bonds. Using our first-principles-determined longitudinal and transverse acoustic-mode Gr\"uneisen parameters, zone-boundary frequencies, and phonon group velocities, we calculate the lattice thermal conductivity using the Debye-Callaway model. The theoretical thermal conductivity is in good agreement with the experimental measurements.

Origin of ferromagnetism and the design principle in phase-change magnetic materials

The intrinsic ferromagnetism (FM) in phase-change (PC) magnetic materials was studied by first-principle calculations and experiments. The doped Fe or other transition metals tend to substitute at the Sb site of Ge${}_{2}$Sb${}_{2}$Te${}_{5}$. An asymmetric coupling, between ${p}_{\mathrm{x}}$ and ${p}_{\mathrm{y}}$ rather than ${p}_{\mathrm{z}}$ states of Te and $d$ states of Fe, creates magnetic moments and holes in the $p$ band. The findings reveal a strong dependence of FM interaction on Fe-Fe pair distances and crystallographic orientation. A design principle with three criteria is proposed for exploring new PC magnetic materials.

Electronic structure of single crystal UPd 3, UGe 2, and USb 2 from hard X-ray and angle-resolved photoelectron spectroscopy

Electronic structure of single crystal UPd 3, UGe 2, and USb 2 has been measured from hard X-ray photoelectron spectroscopy (HAXPES) with 7.6 keV photons at the European Synchrotron Radiation Facility (ESRF). Lower photon energy angle-resolved photoelectron spectroscopy (ARPES) was also performed at the Synchrotron Radiation Center (SRC). Herein the following results are presented: (i) ARPES results demonstrate hybridization between the U 5f and Pd 4d electrons within UPd 3. (ii) The greatly reduced surface sensitivity of HAXPES enabled observation of the bulk core levels in spite of surface oxidation. Photoelectron mean-free-path versus oxide layer thickness considerations were used to model the effectiveness of HAXPES for probing bulk features of in-air cleaved samples. (iii) Two distinct features separated by 800 meV were observed for the Sb 3d core level. These two features are attributed to manifestations of two distinct Sb sites within the USb 2 single crystal as supported by consideration of interatomic distances and enthalpy-of-formation. (iv) Doniach–Sunjic line shape analysis of core level spectral features revealed correlations between asymmetry coefficients and 5f localization.

Doping Effects on the Thermoelectric Properties of Cu<SUB>3</SUB>SbSe<SUB>4</SUB>

We present the first systematic doping study on the ternary semiconductor Cu3SbSe 4 . We have developed a novel synthesis procedure that produces high-quality polycrystalline samples with hole concentrations an order of magnitude lower than have been reported for the undoped compound. The hole concentration can be increased by adding small amounts of either Ge or Sn on the Sb site. The power factor increases with increasing doping, reaching a maximum value of 16 W/cmK^2 . The thermoelectric properties are optimized for the 2% Sn doped compound which has ZT=0.72 at 630K, rivaling that of state-of-the-art thermoelectric materials in this temperature range.

New stannite-like p-type thermoelectric material Cu3SbSe4

The ternary chalcogenide of Cu3SbSe4 is demonstrated to be a novel p-type thermoelectric material, by doping Sn in the Sb site. The figure of merit (ZT) in Cu3Sb0.975Sn0.025Se4 reaches 0.75 at 673 K. Such excellent thermoelectric properties are attributed to the crystal structure of Cu3SbSe4, consisting of the three-dimensional Cu/Se framework acting as the hole conduction pathway and the [SbSe4] tetrahedra. The Cu/Se framework is suitable to tune the electrical conductivity by doping. The insertion of tetrahedral [SbSe4] causes a more distorted diamond-like structure, providing a relatively lower lattice thermal conductivity and a relatively large Seebeck coefficient. The origin of the structure–electrical property relationship and ZT enhancement by Sn doping is elucidated.

High thermoelectric figure of merit in the Cu3SbSe4-Cu3SbS4 solid solution

We report thermoelectric properties of selected compounds from the Cu3SbSe4-Cu3SbS4 system. Additional phonon scattering due to the disordered arrangement of Se and S atoms reduces the lattice thermal conductivity to near its minimum possible value at high temperature. The hole concentration is optimized at approximately 2.0 × 1020 cm−3 by doping with 3% Ge on the Sb site. Compounds of the form Cu3Sb1−yGeySe4−xSx (x = 0.8 and 1.2, y = 0.02 and 0.03) all have dimensionless thermoelectric figure of merit in excess of 0.8 at 650 K, with a maximum value of 0.89 for x = 1.2, y = 0.03.

First-principles study on half-metallic properties of CuHg2 Ti-type alloys FeV2 Z (Z = Si, Ge, As, Sb)

The first-principles calculation within density-functional theory is used to investigate the electronic structures and magnetic properties in CuHg2Ti-type alloys FeV2Z (Z = Si, Ge, As, Sb). FeV2Sb is predicted to be half-metal and the energy gap lies in the majority-spin bands. Though FeV2Si, FeV2Ge, and FeV2As show no half-metallicity, their spin polarization of conduction electrons is around 99.7, 99.0, and 97.8%, respectively. The investigation of volume distortion shows that over 99% spin-polarization can be retained in the range of 5.70–6.05 A for FeV2Si, 5.85–6.05 A for FeV2Ge, and 5.90–6.00 A for FeV2As, respectively. FeV2Sb can keep half-metallicity from 6.00 to 6.15 A. We studied the effects of defects at transition-metal sites and Ge, As, and vacancy substitution for Sb atom on the electronic structures in FeV2Sb, and the results indicate that half-metallicity in FeV2Sb is not sensitive to substitutions for Sb sites.

Spin-polarized first-principles study of ferromagnetism in zinc-blende In1−xMnxSb

First-principles calculations based on the density functional theory are performed to study the structural properties, spin-polarized electronic band structures, density of states and magnetic properties of the zinc blende In1−xMnxSb (x = 0.125, 0.25, 0.50, 0.75, 1.0). The calculated lattice constants of In1−xMnxSb obey the Vegard’s law with a marginal upward bowing. With the increase of Mn concentration in In1−xMnxSb, a transition from the semi-metallic to the half-metallic behavior happens such that the majority-spin valence states crosses the Fermi level and the minority-spin states have a gap at the Fermi level. A large exchange splitting (∼ 4 eV) is observed between Mn 3d states of the majority-spins and the minority-spins. The total magnetic moment of In1−xMnxSb half-metallic ferromagnets per Mn atom basis is 4μB. The total magnetic moment per Mn atom indicate that Mn atoms act as acceptors in InSb and contribute to holes in the lattice of InSb. Due to p-d hybridization, the free space charge of Mn reduces that results a loss in its magnetic moment. The loss in the magnetic moment of the Mn atoms is converted into a small local magnetic moments on the In and Sb sites.

A new hydrazine-bridged thioantimonate Mn2Sb4S8(N2H4)2: Synthesis, structure, optical and magnetic properties

Abstract A new thioantimonate [Mn 2 Sb 4 S 8 (N 2 H 4 ) 2 ] ( 1 ) was solvothermally synthesized by reacting Mn and Sb 2 S 3 with S in hydrazine monohydrate solution. The single crystal structure analysis revealed that all Mn sites adopt distorted octahedral shapes and all Sb sites have either four-fold coordinated SbS 4 configurations or square pyramidal SbS 5 geometry. These polyhedra units are interconnected into two-dimensional layers via the sharing S atoms. These layers are further bridged into a novel three-dimensional framework by intra- and inter-layer hydrazine ligands. The compound 1 is a semiconductor with a band gap of 1.59 eV and displays paramagnetic behavior at high temperature and switches to antiferromagnetic ordering at 40 K.

Effects of Bi doping on the thermoelectric properties of β-Zn4Sb3

The thermoelectric properties of Bi-doped compounds (Zn1−xBix)4Sb3 (x=0,0.0025,0.005,0.01) were studied experimentally as well as theoretically. The results indicate that low-temperature (T<300 K) thermal conductivity of moderately doped (Zn0.9975Bi0.0025)4Sb3 reduces remarkably as compared with that of Zn4Sb3 due to enhanced phonon scattering of impurity (dopant). Electrical resistivity and Seebeck coefficient increase monotonically with increase in the Bi content resulting mainly from decrease in carrier concentration. Moreover, first-principle calculations were performed on the occupation options of Bi atoms in β-Zn4Sb3, which show that Bi will preferentially occupy the Zn sites and not Sb sites and act as donors, being consistent with the experimental observations. In addition, the lightly doped compound (Zn0.9975Bi0.0025)4Sb3 exhibits the best thermoelectric performance due to the improvement in both its thermal conductivity and Seebeck coefficient, whose figure of merit, ZT, is about 1.5 times larger than that of β-Zn4Sb3 at 300 K.

NMR studies in the half-Heusler type compound YbPtSb

121Sb and 19BPt nuclear magnetic resonance (NMR) has been studied in the half-Heusler type compound YbPtSb to obtain information on local magnetic behavior. The characteristics of the localized 4f spins are observed in the Cuire-Weiss type behavior of the Knight shifts K for both 121Sb and 19BPt. From the slope of K-χ plots we estimated hyperfine coupling constants of -3.8 and -4.6 kOe/μB at Sb and Pt sites, respectively. It was found that the spin-echo decay rate 1/T2 of 121Sb shows a clear peaks at 10 K. Similar tendency was also observed in case of 19BPt. However, static properties do not show any anomalies near 10 K.

Thermoelectric Properties of the Pseudobinary Alloy (Ag0.365Sb0.558Te)0.975(GeTe)0.025 Prepared by Spark Plasma Sintering

Ag-Sb-Te-Ge-based alloys have received great attention in recent years. In the present work we prepared the pseudobinary alloy (Ag 0.365 Sb 0.558 Te) 0.975 (GeTe) o . o25 using spark plasma sintering and evaluated its thermoelectric (TE) properties over the temperature range from 318 K to 551 K. Rietveld analysis revealed that about 1.3 at.% Ge atoms occupy the Sb sites and that the alloy exhibits the same crystal structure as AgSbTe 2 . By using back-scattered electron imaging, we observed two instead of one phase in the sample. The small white AgSbTe 2 chunks embedded in the matrix can substantially scatter phonons. Compared with the transport properties of Ag 0.365 Sb 0.558 Te, we obtained a slightly increased Seebeck coefficient and reduced thermal conductivity without sacrificing electrical conductivity. The highest TE figure of merit, ZT, was 0.69 at 551 K, whereas that of the ternary alloy Ag 0.365 Sb 0.558 Te was 0.61 at the corresponding temperature, suggesting that (Ag 0.365 Sb 0.558 Te) 0.975 (GeTe) 0.025 has the potential to improve TE performance with optimization of its chemical composition.

I8Sb10Ge36

Single crystals of the title compound, octa­iodide deca­anti­monate hexa­tria­conta­germanide, were grown by chemical transport reactions. The structure is isotypic with the analogous clathrates-I. In this structure, the (Ge,Sb)46 framework consists of statistically occupied Ge and Sb sites that atoms form bonds in a distorted tetra­hedral arrangement. They form polyhedra that are covalently bonded to each other by shared faces. There are two polyhedra of different sizes, viz. a (Ge,Sb)20 dodeca­hedron and a (Ge,Sb)24 tetra­cosa­hedron in a 1:3 ratio. The guest atom (iodine) resides inside these polyhedra with symmetry m3 (Wyckoff position 2a) and 2m (Wyckoff position 2d), respectively.

Atomic order and magnetization distribution in the half metallic and nearly half metallicC1b compounds NiMnSb and PdMnSb

Polarized neutron diffraction has been used to study the magnetization distribution in twoisostructural inter-metallic compounds NiMnSb and PdMnSb. Band structure calculationshave predicted that whereas the former should be a half metallic ferromagnet the lattershould not. Measurements made at 5 K on different crystals show that disorder canoccur between the A (Mn) and B (Sb) sites in both alloys and in the case ofNiMnSb, by partial occupation of the void D sites by Ni. In all the crystals most ofthe moment was found on the Mn atoms in the A sites; in NiMnSb it is due tospin only but in PdMnSb there is evidence for a significant orbital contribution (g = 2.22). The magnitudes of the moments associated with each atom are infair agreement with the theoretical values; however, the distribution ofmagnetization around the Mn atoms is found to have nearly spherical symmetry (40% eg) ratherthan the 50% eg character expected from the band structure.

U235nuclear relaxation rates in an itinerant antiferromagnetUSb2

$^{235}$U nuclear spin-lattice ($T_1^{-1}$) and spin-spin ($T_2^{-1}$) relaxation rates in the itinerant antiferromagnet USb$_2$ are reported as a function of temperature in zero field. The heating effect from the intense rf pulses that are necessary for the $^{235}$U NMR results in unusual complex thermal recovery of the nuclear magnetization which does not allow measuring $T_1^{-1}$ directly. By implementing an indirect method, however, we successfully extracted $T_1^{-1}$ of the $^{235}$U. We find that the temperature dependence of $T_1^{-1}$ for both $^{235}$U and $^{121}$Sb follows the power law ($\propto T^n$) with the small exponent $n=0.3$ suggesting that the same relaxation mechanism dominates the on-site and the ligand nuclei, but an anomaly at 5 K was observed, possibly due to the change in the transferred hyperfine coupling on the Sb site.